[web] Remove extraneous containerPosition prop from MessageActionButtons

Summary: We can get containerPosition from mouseOverMessagePosition instead of passing it in directly as a prop

Test Plan: NA, flow

Reviewers: def-au1t, palys-swm, varun, benschac

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3311